Inter-rat handover control using empty gre packets

ABSTRACT

Empty GRE packets are used to provide in-order delivery of data packets for a session to a UE during inter-RAT handover. In particular, an empty GRE packet sent from a source gateway in a source RAN to a target gateway in a target RAN indicates to the target gateway the end of forwarded data packets from the source gateway. The target gateway sends data packets received from the source gateway to the UE until the empty GRE packet is received. Upon receipt of the empty GRE packet, the target gateway begins sending data packets received directly from a home network gateway to the UE.

The invention disclosed herein generally relates to handover of a mobilestation, and more particularly to in-order delivery of data packetsduring inter-RAT handover using empty Generic Routing Encapsulation(GRE) packets.

BACKGROUND

The 3^(rd) Generation Partnership Project (3GPP) oversees and governs3^(rd) Generation (3G) networks, including 3G Long Term Evolution (LTE)networks. 3G LTE provides mobile broadband to User Equipment (UEs)within the 3G LTE network at higher data rates than generally availablewith other networks. For example, the air interface for 3G LTE, EvolvedUniversal Mobile Telecommunication System (UMTS) Terrestrial RadioAccess Network (E-UTRAN), utilizes multi-antenna and multi-user codingtechniques to achieve downlink data rates of 100 s of Mbps and uplinkdata rates of 10 s of Mbps.

In LTE, user mobility is controlled by the network with assistance fromthe UE. Handover decisions, as well as the choices for the target celland technology (when applicable), are made by the current serving eNodeB(equivalent to Base Station in 2G/3G network) based on measurements madeby the eNodeB, and based on measurements reported by the UE to theeNodeB. Due to the nature of E-UTRAN, the number of packets bufferedbefore scheduled transmissions occur may not be negligible. For thatreason, packet forwarding mechanisms may be used (when applicable)between a source node and a target node so as to limit packet lossduring handover from the source node to the target node.

Due to various delays, e.g., those caused by the forwarding process, thetarget node may receive forwarded data packets after receivingpost-handover data packets. Such delays may cause the target node todeliver data packets to the UE out of order. Procedures currently existto guarantee in-order packet delivery to the UE during handover of a UEbetween network nodes within the same Radio Access Network (RAN) and/orassociated with the same Radio Access Technology (RAT). However, becauseno such procedures exist for handover of a UE between network nodesassociated with some RATs, i.e., handover from 3GPP to HRPD (High RatePacket Data), there is a risk of out-of-order packet delivery.

SUMMARY

Embodiments of the invention disclosed herein use empty GRE (GenericRouting Encapsulation) packets to provide in-order delivery of datapackets for a session to a UE (User Equipment) during inter-RAT (RadioAccess Technology) handover. In particular, an empty GRE packet sentfrom a source gateway in a source RAN (Radio Access Network) to a targetgateway in a target RAN indicates to the target gateway the end offorwarded data packets from the source gateway. The target gateway sendsdata packets received from the source gateway to the UE until the emptyGRE packet is received. Upon receipt of the empty GRE packet, the targetgateway begins sending data packets received directly from a homenetwork gateway to the UE.

The network gateway, source gateway, and target gateway each play a partin implementing the inter-RAT handover described herein. After receivinghandover instructions, the network gateway sends an end-marker packet tothe source gateway to indicate the end of the data packets being sent bythe network gateway to the source gateway. If the session includes morethan one bearer stream, the network gateway sends an end-marker packetfor each bearer stream. The network gateway subsequently sends one ormore data packets for the session directly to the target gateway, wherethe direct data packets are sequentially ordered relative to the datapackets sent to the source gateway. In some embodiments, the networkgateway may also send an empty GRE packet to the target gateway beforesending the data packets to the target gateway. The empty GRE packetindicates the start of the transmission of data packets for the sessionfrom the network gateway directly to the target gateway.

The source gateway forwards the data packets to the target gateway inthe target RAN. Responsive to receiving an end-marker packet, the sourcegateway generates and sends an empty GRE packet to the target gateway.If the session includes multiple bearer streams, the source gatewaysends the empty GRE packet after receiving the end-marker packet foreach bearer stream. The empty GRE packet indicates to the target gatewaythe end of the forwarded packets for the session.

The target gateway sends data packets to the UE based on the receiveddata packets and the received empty GRE packet(s). More particularly,the target gateway sends the forwarded data packets received from thesource gateway to the UE. Responsive to receiving the empty GRE packetfrom the source gateway, the target gateway sends the data packetsreceived directly from the network gateway to the UE.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a 3GPP network and HRPD networkinterfacing with a mobile station.

FIG. 2 shows a simplified block diagram of the source and targetnetworks interfacing with the mobile station during inter-RAT handoveraccording to one exemplary embodiment disclosed herein.

FIG. 3 shows a block diagram of an exemplary network gateway.

FIG. 4 shows an exemplary method of inter-RAT handover as implemented bythe network gateway of FIG. 3.

FIG. 5 shows a block diagram of an exemplary source gateway.

FIG. 6 shows an exemplary method of inter-RAT handover as implemented bythe source gateway of FIG. 5.

FIG. 7 shows a block diagram of an exemplary target gateway.

FIG. 8 shows an exemplary method of inter-RAT handover as implemented bythe target gateway of FIG. 7.

FIG. 9 shows another exemplary method of inter-RAT handover asimplemented by the network gateway of FIG. 3.

FIG. 10 shows another exemplary method of inter-RAT handover asimplemented by the source gateway of FIG. 5.

FIG. 11 shows another exemplary method of inter-RAT handover asimplemented by the target gateway of FIG. 7.

FIG. 12 shows an example of inter-RAT handover between 3GPP and HRPDnetworks according to one exemplary embodiment disclosed herein.

FIG. 13 shows an example of a call flow diagram for the inter-RAThandover of FIG. 2.

DETAILED DESCRIPTION

The embodiments disclosed herein use empty GRE (Generic RoutingEncapsulation) packets to deliver packets for a session in-order to a UE(User Equipment) during inter-RAT (Radio Access Technology) handover. Inparticular, an empty GRE packet sent from a source gateway in a sourceRAN (Radio Access Network) to a target gateway in a target RAN indicatesthe end of the data packets from the source gateway. The target gatewaysends data packets received from the source gateway to the UE until theempty GRE packet is received. After receipt of the empty GRE packet, thetarget gateway sends data packets received directly from the networkgateway to the UE. While the embodiments are described herein in termsof 3GPP and HRPD networks, the embodiments of the invention disclosedherein may generally apply to any downlink inter-RAT handover.

Before describing further details, the following first generallydescribes inter-RAT handover. FIG. 1 shows a wireless network includingelements associated with a home mobile network, 3GPP RAN, and HRPD RANduring inter-RAT handover of the UE between 3GPP and HRPD. The wirelessnetwork applies to both roaming and non-roaming scenarios, where the S5interface between the Packet Data Network (PDN) Gateway (PGW) and theServing Gateway (SGW) applies to non-roaming scenarios, and the S8interface between the PGW and SGW applies to roaming scenarios. It willbe appreciated that FIG. 1 omits various elements, e.g., the PCRF, AAAservers, etc., for simplicity.

The home mobile network represents one or more external networks, andincludes an IP node, a Home Subscriber Server (HSS), and a PGW. The IPnode provides data associated with IP services, e.g., IMS, PSS, etc., tothe PGW. The HSS comprises a central database containing user-relatedand subscription-related information. In addition, the HSS providesmobility management, call and session establishment support, userauthentication, and access authorization. The PGW provides connectivitybetween the UE and the home mobile network. Further, the PGW serves asan anchor for mobility between 3GPP and non-3GPP technologies.

The PGW provides data packets for a session to the SGW via the S5(non-roaming) or S8 (roaming) interface. The SGW routes GTP data packetsto an eNodeB of the E-UTRAN via the S1-U interface for transmission tothe UE according to the 3GPP standard. After the eNodeB makes thedecision to handover the UE to a non-3GPP network, e.g., the HRPDnetwork, the eNodeB returns any received GTP data packets for thesession back to the SGW. The SGW forwards the returned data packets tothe HSGW via the S103 interface as GRE data packets.

To complete the handover, the PGW sends a GTP-U end-marker packet to theSGW to indicate the end of the data packets being sent to the SGW.Subsequently, the PGW sends GRE data packets for the session to the HSGWvia the S2a interface. After the downlink path is switched at the PGW,forwarded data packets on the S103 interface and GRE data packets on theS2a interface may arrive interchanged at the HSGW, which may hinder orotherwise prevent the HSGW from delivering the data packets for thesession via the HRPD AN to the UE in order.

One possible solution to this problem is to insert sequence numbers inthe header of each data packet. While such sequence numbers would enablethe HSGW to determine the correct order for the data packets, using suchsequence numbers also undesirably increases the overhead and signalprocessing associated with the transmission of each data packet.

The inter-RAT handover described herein solves this problem by using anempty GRE packet to indicate the end of the forwarded packets to theHSGW. FIG. 2 shows a simplified block diagram of the wireless networkimplementing the inter-RAT handover described herein. The wirelessnetwork includes a network gateway 100 in a home mobile network, asource gateway 200 in a source network, and a target gateway 300 in atarget network. The source network sends data packets from the SGW 200to the UE 400 via the source AN 260. The target network sends datapackets from the target gateway 300 to the UE 400 via the target AN 360.While not required, examples of the network gateway 100, source gateway200, source AN 260, target gateway 300, and target AN 360 respectivelycomprise the PGW, SGW, eNodeB/E-UTRAN, HSGW, and HRPD AN shown in FIG.1.

SGW 200 sends an empty GRE packet to the HSGW 300 based on an end-markerpacket originating at the PGW 100 and returned to the SGW 200 from thesource AN 260. The empty GRE packet indicates to the HSGW 300 the end ofthe data packets from the SGW 200. The HSGW 300 sends data packetsreceived from the SGW 200 to the UE 400 until the empty GRE packet isreceived. Upon receipt of the empty GRE packet, HSGW 300 begins sendingdata packets received directly from the PGW 100 to the UE 400.

FIGS. 3 and 4 respectively describe handover operations from theperspective of the PGW 100 and a method 180 implemented by the PGW 100.PGW 100 comprises a transmitter 110 and a control unit 120. Transmitter110 sends source RAN data packets, e.g., GTP data packets, for thesession to the SGW 200 according to 3GPP protocols (block 182). Controlunit 120 generally controls the operation of the PGW 100, and includes apacket router 122 to control packet transmissions before, during, andafter handover. More particularly, after the transmitter 110 sends thelast GTP data packet, packet router 122 controls the transmitter 110 tosend an end-marker packet, e.g., a GTP-U end-marker packet, indicatingthe end of the GTP data packets to the SGW 200 for the session (block184). If the session includes multiple bearer streams, the packet router122 controls the transmitter to send an end-marker packet for eachbearer stream.

After the end-marker packet(s) are sent to the SGW 200, the packetrouter 122 controls the transmitter 110 to send target RAN data packetsfor the session, e.g., GRE data packets, to the UE 400 according to HRPDprotocols. In some embodiments, the packet router 122 generates an emptyGRE packet and controls the transmitter 110 to send the empty GRE packetto the HSGW 300 (block 186) after sending the end-marker packet(s) tothe SGW 200 and before sending the GRE data packets to the HSGW 300. Theempty GRE packet indicates the beginning of the subsequent transmissionof the GRE data packets for the session to the HSGW 300. After sendingthe empty GRE packet, the transmitter 110 sends the subsequent GRE datapackets for the session to the HSGW 300 (block 188). The subsequent GREdata packets are sequentially ordered relative to the GTP data packets.

FIGS. 5 and 6 respectively describe the handover operations from theperspective of the SGW 200 and a method 280 implemented by the SGW 200.The SGW 200 comprises a receiver 210, control unit 220, and transmitter230. Receiver 210 receives source RAN data packets, e.g., GTP datapackets, for the session from the PGW 100. During handover, the receiver210 also receives an end-marker packet for each of the one or morebearer streams of the session. Control unit 220 comprises a packetrouter 222 configured to direct the received GTP data packets to thetransmitter 230 for transmission to the eNodeB 260. Before handover, theeNodeB 260 transmits the GTP data packets to the UE 400. After eNodeB260 breaks the connection with the UE 400 during handover, the eNodeB260 returns any received GTP data packets, including any GTP-Uend-marker packet(s) to the SGW 200.

Responsive to receiving returned GTP data packets from the eNodeB 260,the packet router 222 controls the transmitter 230 to forward the datapackets as target RAN data packets, e.g., GRE data packets, to the HSGW300 (block 282). In addition, packet router 222 generates an empty GREpacket responsive to the end-marker packet, e.g., the end-marker packetreturned by the eNodeB 260 (blocks 284, 286). If the session includesmultiple bearer streams, the packet router 222 generates the empty GREpacket responsive to receiving an end-marker packet for each bearerstream. Subsequently, the packet router 222 controls the transmitter 230to send the empty GRE packet to the HSGW 300 to indicate to the HSGW 300the end of the GRE data packets from the SGW 200 (block 288).

FIGS. 7 and 8 respectively describe the handover operations from theperspective of the HSGW 300 and a method implemented by the HSGW 300.HSGW 300 includes a receiver 310, control unit 320, and transmitter 330.During handover, receiver 310 receives target RAN data packets for thesession, e.g., GRE data packets, from both the SGW 200 and the PGW 100,where the GRE data packets received from the SGW 200 contain payloadthat is the same as the payload of the GTP data packets originating atthe PGW 100 and forwarded by the SGW 200 (block 382). Until the HSGW 300receives the empty GRE packet from the SGW 200 (block 384), packetrouter 322 controls the transmitter 320 to send the forwarded datapackets from the SGW 200 to the UE 400 via the Access Node (AN) 360(blocks 384, 386). Once the HSGW 300 receives the empty GRE packet fromthe SGW 200, the packet router 322 controls the transmitter to send thedata packets received directly from the PGW 100 to the UE 400 via AN 360(blocks 384, 388).

In some embodiments, the HSGW 300 may also include a buffer 340. Buffer340 buffers the data packets received directly from the PGW 100 untilreceiver 310 receives the empty GRE packet from the SGW 200. Uponreceipt of the empty GRE packet, the packet router 322 controls thetransmitter 330 to send the buffered data packets to the UE 400. Oncethe buffer is empty 340, the packet router 322 controls the transmitter330 to send the data packets received from the PGW 100.

The HSGW 300 may also include a timer 350 to ensure that the buffereddata packets are eventually delivered to the UE 400, even if the emptyGRE packet is never received. For example, the packet router 322 maycontrol the transmitter 330 to send the buffered data packets uponexpiration of the timer 350. Thus, if the empty GRE packet is lost ordamaged, the HSGW 300 will still send the buffered data packets uponexpiration of the timer. The timer 350 may be set based on an expectedduration of the handover process. In one embodiment, timer 350 startsresponsive to receipt of an empty GRE packet from the PGW 100. In antherembodiment, the timer 350 starts responsive to receipt of the first datapacket from the PGW 100.

The embodiments described above rely on empty GRE packets to indicatethe end of the session data packets being sent from the SGW 200 to theHSGW 300. In some instances, an empty GRE packet may also be used toindicate the beginning of session data packets being sent from the PGW100 to the HSGW 300 during handover. Other embodiments may also oralternatively use one or more sequence numbers in a header of an emptypacket or a data packet to indicate the end and/or beginning of the datapackets. FIGS. 9-11 provide exemplary methods for a PGW 100, SGW 200,and HSGW 300, respectively, when sequence numbers are used to indicatethe beginning and/or end of data packet transmissions.

FIG. 9 shows an exemplary method 190 from the perspective of the PGW 100for handling inter-RAT handover using sequence numbers. Transmitter 110sends GTP data packets for the session to the SGW 200 (block 192). Afterthe transmitter 110 sends the last GTP data packet, the packet router122 controls the transmitter 110 to send an end-marker packet indicatingthe end of the GTP data packets for the session (block 194). The headerof the end-marker packet includes a sequence number field containing afirst sequence number. If the session includes multiple bearer streams,the packet router 122 controls the transmitter to send an end-markerpacket for each bearer stream, where each end-marker packet includes afirst sequence number in the header. In one embodiment, the end-markerpacket for each bearer stream includes a different sequence number. Itwill be appreciated, however, that some or all of the end-marker packetsmay include the same sequence number.

After sending the end-marker packet, the packet router 122 controls thetransmitter 110 to send an initial GRE data packet with a secondsequence number in the header to the HSGW 300 (block 196). The secondsequence number indicates the beginning of the GRE data packets beingsent from the PGW 100 directly to the HSGW 300. The initial data packetmay contain payload data in the body of the packet. Alternatively, theinitial data packet may comprise an empty GRE packet. After sending theinitial data packet, transmitter 110 sends the subsequent GRE datapackets for the session to the HSGW (block 198). The subsequent GRE datapackets are sequentially ordered relative to the GTP data packets sentto the SGW 200.

FIG. 10 shows an exemplary method 290 from the perspective of the SGW200 for handling inter-RAT handover using sequence numbers. Responsiveto receiving returned GTP data packets from the eNodeB, the packetrouter 222 controls the transmitter 230 to forward the data to the HSGW300 in GRE data packets (block 292). In addition, packet router 222generates an empty GRE packet responsive to an end-marker packet, e.g.,the end-marker packet returned from the eNodeB 260 (blocks 294, 296).The header of the empty GRE packet includes the sequence number in thereturned end-marker packet. If the session includes multiple bearerstreams, an end-marker packet containing a sequence number is receivedfor each bearer stream. The packet router 222 selects one of thesequence numbers in the received end-marker packets, e.g., the largestsequence number, and generates the empty GRE packet with the selectedsequence number. After the end-marker packet for each bearer stream isreceived, the packet router 222 controls the transmitter 230 to send theempty GRE packet to the HSGW 300 to indicate to the HSGW 300 the end ofthe data packets for the session sent by the SGW 200 (block 298).

FIG. 11 shows an exemplary method 390 from the perspective of the HSGW300 for handling inter-RAT handover using sequence numbers. Duringhandover, receiver 310 receives GRE data packets for the session fromboth the SGW 200 and the PGW 100, where the GRE data packets receivedfrom the SGW 200 contain payload that is the same as the payload of theGTP data packets originating at the PGW 100 and forwarded by the SGW 200(block 392). Until the HSGW 300 receives the empty GRE packet with thesequence number from the SGW 200 (block 384), the packet router 322controls the transmitter 320 to send the forwarded GRE data packets fromthe SGW 200 to the UE 400 via the HRPD AN 360 (blocks 394, 396). Oncethe HSGW 300 receives the empty GRE packet with the sequence number fromthe SGW 200, the packet router 322 controls the transmitter to send theGRE data packets received directly from the PGW 100 to the UE 400 viathe HRPD AN 360 (blocks 394, 398).

In some embodiments, the HSGW 300 receives an empty GRE packet having afirst sequence number from the SGW 200 and an initial data packet havinga second sequence number from the PGW 100. In this case, the HSGW 300determines whether or not the empty GRE packet and initial GRE datapacket are being used to indicate the end and beginning of the datapackets from the respective gateways by comparing the first and secondsequence numbers. If the sequence numbers have the expectedrelationship, e.g., the second sequence number is greater than the firstsequence number, the first and second sequence numbers are equal, etc.,control unit 320 determines that the HSGW 300 has received theindication of the end and start of the data packets from the respectiveSGW 200 and PGW 100. Based on this information, packet router 322determines whether or not to have the transmitter 330 begin sending thedirect data packets from the PGW 100 to the UE 400.

When a buffer 340 and timer 350 are included in the HSGW 300, the packetrouter 322 may also control the transmitter 330 to send the buffereddata packets upon expiration of the timer 350. Thus, if the empty GREpacket is lost or damaged, the HSGW 300 will still send the buffereddata packets upon expiration of the timer. The timer 350 may be setbased on an expected duration of the handover process. In oneembodiment, timer 350 starts responsive to receipt of the empty GREpacket with the sequence number from the PGW 100. In anther embodiment,the timer 350 starts responsive to receipt of the first data packetcontaining a sequence number from the PGW 100.

When the empty GRE packet sent from the SGW 200 includes a sequencenumber, the previously sent data packets generally do not include asequence number. Similarly, when the initial data packet sent form thePGW 100 includes a sequence number, the subsequently sent data packetsgenerally do not include sequence numbers. It will be appreciated,however, that the embodiments disclosed herein do not preclude the useof sequence numbers in the other data packets.

FIGS. 12 and 13 respectively show an exemplary block diagram and callflow diagram implementing handover from a 3GPP network to an HRPDnetwork according to one exemplary embodiment. PGW 100 sends userpayload packets a, b, and c to the SGW 200 via the S5/58 GTP tunnel. SGW200 sends the user payload packets a, b, and c to the eNodeB 260 in theE-UTRAN via the S1-U GTP tunnel. Because eNodeB 260 has alreadydisconnected from the UE 400 and the HRPD AN 360 has connected to the UE400, the eNodeB 260 returns the user payload packets a, b, and c to theSGW 200 via an indirect GTP tunnel.

After sending the last data packet (data packet c), the PGW 100 sends anend-marker packet for each bearer stream (call flow item 14 c.i). In theexample in FIG. 12, there is only one bearer stream, and the associatedend-marker packet includes sequence number 100. SGW 200 sends theend-marker packet to the eNodeB 260 (call flow item 14 c.ii), whichreturns it to the SGW 200 as part of the data packet forwarding process(call flow item 14 c.iii). Responsive to the returned end-marker packet,the SGW 200 generates an empty GRE packet that includes sequence number100. After SGW 200 forwards the user payload packets a, b, and c to theHSGW 300 via the S103 GRE tunnel, the SGW 200 sends the empty GRE packetto the HSGW (call flow item 14 c.v).

After PGW 100 sends the end-marker packet(s) to SGW 200 (call flow item14 c.i), the PGW 100 sends an initial packet followed by user payloadpackets d, e, and f to the HSGW 300 via the S2a GRE tunnel. In theexample shown in FIG. 12, the initial packet comprises an empty GREpacket that includes sequence number 101 (call flow item 14 c.iv). TheHSGW 300 receives user payload packets a, b, and c and sends them to theHRPD AN 360 for transmission to the UE 400. Upon receipt of the emptyGRE packet, the control unit 320 compares the sequence number in theempty GRE packet received from the SGW 200 to the sequence number in theempty GRE packet received from the PGW 100. Because sequence number 101is greater than sequence number 100, as expected by the HSGW 300, theHSGW sends user payload packets d, e, and f to the HRPD AN 360. If theHSGW 300 receives user payload packets d, e, or f before receiving theempty GRE packet with sequence number 100, the HSGW 300 buffers userpayload packets d, e, and/or f in buffer 340 until the empty GRE packetis received, and sends the buffered user payload packets to the HRPD AN360 after the empty GRE packet is received. After the buffer is emptied,the HSGW 300 sends user payload packets received from the PGW 100 viathe S2a GRE tunnel in the order they are received.

While the embodiments are generally described herein in terms ofhandover of a UE 400 from 3GPP to HRPD, it will be appreciated that thevarious embodiments and details also apply to handover of a UE 400 fromHRPD to 3GPP, where the source, target, and network gatewaysrespectively comprise the HSGW 300, SGW 200, and PGW 100. In particular,an empty GRE packet sent from the HSGW 300 to the SGW 200 indicates theend of the data packets from the HSGW 300. The SGW 200 sends datapackets received from the HSGW 300 to the UE 400 until the empty GREpacket is received. After receipt of the empty GRE packet, the SGW 200sends data packets received directly from the PGW 100 to the UE 400.

The embodiments disclosed herein facilitate inter-RAT handover byensuring in-order delivery of data packets to the UE during thehandover. The inter-RAT handover disclosed herein may, of course, becarried out in other ways than those specifically set forth hereinwithout departing from essential characteristics of the invention. Thepresent embodiments are to be considered in all respects as illustrativeand not restrictive, and all changes coming within the meaning andequivalency range of the appended claims are intended to be embracedtherein.

1. A method implemented by a target gateway in a target Radio AccessNetwork (RAN) of delivering data packets for a session to a mobilestation during inter-RAT (Radio Access Technology) handover, the methodcomprising: receiving one or more forwarded packets for the session froma source gateway in a source RAN; receiving a first empty GRE (GenericRouting Encapsulation) packet from the source gateway indicating the endof the forwarded packets for the session; sending the forwarded packetsto the mobile station; receiving one or more direct packets for thesession from a network gateway, said direct packets sequentially orderedrelative to the forwarded packets; and sending the direct packets to themobile station responsive to receiving the first empty GRE packet. 2.The method of claim 1 further comprising starting a timer responsive toreceiving a first one of the direct packets, wherein sending the directpackets comprises sending the direct packets to the mobile stationresponsive to expiration of the timer.
 3. The method of claim 1 furthercomprising receiving a second empty GRE packet from the network gatewayindicating the start of the direct packets for the session.
 4. Themethod of claim 3 further comprising starting a timer responsive toreceiving the second empty GRE packet, wherein sending the directpackets comprises sending the direct packets to the mobile stationresponsive to expiration of the timer.
 5. The method of claim 1 whereinreceiving the first empty GRE packet comprises receiving the first emptyGRE packet before receiving the direct packets for the session, andwherein sending the direct packets to the mobile station responsive toreceiving the first empty GRE packet comprises sending the directpackets to the mobile station upon receipt of the direct packets at thetarget gateway.
 6. The method of claim 1 further comprising buffering,in the target gateway, the direct packets received from the networkgateway before receipt of the first empty GRE packet, wherein sendingthe direct packets comprises sending the buffered packets afterreceiving the first empty GRE packet.
 7. The method of claim 1 whereinthe source gateway comprises one of a 3^(rd) Generation PartnershipProject (3GPP) serving gateway and a High Rate Packet Data (HRPD)serving gateway, the target gateway comprises the other of the 3GPPserving gateway and the HRPD serving gateway, and the network gatewaycomprises a Packet Data Network (PDN) Gateway (PGW).
 8. A target gatewayin a target Radio Access Network (RAN) to deliver data packets for asession to a mobile station during inter-RAT (Radio Access Technology)handover, said target gateway comprising: a receiver configured to:receive one or more forwarded packets for the session from a sourcegateway in a source RAN; receive a first empty GRE (Generic RoutingEncapsulation) packet from the source gateway indicating the end of theforwarded packets for the session; and receive one or more directpackets for the session from a network gateway, said direct packetssequentially ordered relative to the forwarded packets; a transmitterconfigured to send the forwarded packets to the mobile station; and acontrol unit coupled to the transmitter and comprising a packet routerconfigured to control the transmitter to send the direct packets to themobile station responsive to receipt of the first empty GRE packet. 9.The target gateway of claim 8 further comprising a timer, wherein saidcontrol unit is further configured to start the timer responsive toreceipt of a first one of the direct packets at the receiver, andwherein the packet router controls the transmitter by controlling thetransmitter to send the direct packets to the mobile station responsiveto expiration of the timer.
 10. The target gateway of claim 8 whereinthe receiver is further configured to receive a second empty GRE packetfrom the network gateway indicating the start of the direct packets. 11.The target gateway of claim 10 further comprising a timer, wherein saidcontrol unit is further configured to start the timer responsive toreceipt of the second empty GRE packet, and wherein the packet routercontrols the transmitter by controlling the transmitter to send thedirect packets to the mobile station responsive to expiration of thetimer.
 12. The target gateway of claim 8 wherein when the receiverreceives the first empty GRE packet before receiving the direct packetsfor the session, the packet router controls the transmitter to send thedirect packets to the mobile station upon receipt of the direct packetsby the receiver.
 13. The target gateway of claim 8 further comprising abuffer for buffering the direct packets received before receipt of thefirst empty GRE packet, wherein the packet router controls thetransmitter by controlling the transmitter to send the buffered packetsafter receipt of the first empty GRE packet.
 14. The target gateway ofclaim 8 wherein the source gateway comprises one of a 3^(rd) GenerationPartnership Project (3GPP) serving gateway and a High Rate Packet Data(HRPD) serving gateway, the target gateway comprises the other of the3GPP serving gateway and the HRPD serving gateway, and the networkgateway comprises a Packet Data Network (PDN) Gateway (PGW).
 15. Amethod implemented by a network gateway of downlink data packets for asession to a mobile station during inter-RAT (Radio Access Technology)handover, the method comprising: sending one or more first data packetsfor a bearer stream of the session to a source gateway in a source RAN(Radio Access Network) using a first RAT; sending an end-marker packetindicating the end of the first data packets for the bearer stream ofthe session to the source gateway using the first RAT; sending one ormore second data packets for the session to a target gateway in a targetRAN using a second RAT; and sending an empty GRE (Generic RoutingEncapsulation) packet to the target gateway using the second RAT aftersending the end-marker packet and before sending the second data packetsto indicate the start of the second data packets.
 16. The method ofclaim 15 wherein the session includes a plurality of bearer streamsassociated with the first RAT, each bearer stream having a plurality ofdata packets, and wherein: sending the end-marker packet comprisessending an end-marker packet for each of a plurality of bearer streamsto the source gateway using the first RAT, each end-marker packetindicating the end of the first data packets for the associated bearerstream of the session; and sending the empty GRE packet to the targetgateway comprises sending the empty GRE packet to the target gatewayusing the second RAT after sending the end-marker packet for each bearerstream and before sending the second data packets to indicate the startof the second data packets.
 17. The method of claim 15 wherein thesource gateway comprises one of a 3 ^(rd) Generation Partnership Project(3GPP) serving gateway and a High Rate Packet Data (HRPD) servinggateway, the target gateway comprises the other of the 3GPP servinggateway and the HRPD serving gateway, and the network gateway comprisesa Packet Data Network (PDN) Gateway (PGW).
 18. A network gateway todeliver data packets for a session to a mobile station during inter-RAT(Radio Access Technology) handover, the network gateway comprising: atransmitter configured to: send one or more first data packets for abearer stream of the session to a source gateway in a source RAN (RadioAccess Network) using a first RAT; send an end-marker packet indicatingthe end of the first data packets for the bearer stream of the sessionto the source gateway using the first RAT; send one or more second datapackets for the session to a target gateway in a target RAN using asecond RAT; and send an empty GRE (Generic Routing Encapsulation) packetto the target gateway using the second RAT; and a control unit coupledto the transmitter and comprising a packet router configured to controlthe transmitter to send the empty GRE packet after sending theend-marker packet and before sending the second data packets.
 19. Thenetwork gateway of claim 18 wherein the session includes a plurality ofbearer streams associated with the first RAT, each bearer stream havinga plurality of data packets, wherein the transmitter sends theend-marker packet by sending an end-marker packet for each bearer streamto the source gateway using the first RAT, each end-marker packetindicating the end of the first data packets for the associated bearerstream of the session; and wherein the packet router controls thetransmitter by controlling the transmitter to send the empty GRE packetafter sending the end-marker packet for each bearer stream and beforesending the second data packets to indicate the start of the second datapackets.
 20. The network gateway of claim 18 wherein the source gatewaycomprises one of a 3^(rd) Generation Partnership Project (3GPP) servinggateway and a High Rate Packet Data (HRPD) serving gateway, the targetgateway comprises the other of the 3GPP serving gateway and the HRPDserving gateway, and the network gateway comprises a Packet Data Network(PDN) Gateway (PGW).
 21. A method implemented by a source gateway in asource Radio Access Network (RAN) of delivering data packets for asession to a mobile station during inter-RAT (Radio Access Technology)handover, the method comprising: receiving one or more data packets fora bearer stream of the session from a network gateway; receiving anend-marker packet indicating the end of the data packets for the bearerstream of the session; forwarding data packets to a target gateway in atarget RAN; generating an empty GRE (Generic Routing Encapsulation)packet responsive to the received end-marker packet; and sending theempty GRE packet to the target gateway after receiving the end-markerpacket.
 22. The method of claim 21 wherein receiving the end-markerpacket comprises receiving the end-marker packet for each of a pluralityof bearer streams of the session, wherein sending the empty GRE packetcomprises sending the empty GRE packet after receiving the end-makerpacket for each bearer stream.
 23. The method of claim 21 whereinreceiving the end-marker packet comprises receiving the end-markerpacket sent to a source access node in the source RAN and returned tothe source gateway by the source access node, and wherein generating theempty GRE packet comprises generating the empty GRE packet responsive tothe end-marker packet returned from the source access node.
 24. Themethod of claim 21 wherein the source gateway comprises one of a 3 ^(rd)Generation Partnership Project (3GPP) serving gateway and a High RatePacket Data (HRPD) serving gateway, the target gateway comprises theother of the 3GPP serving gateway and the HRPD serving gateway, and thenetwork gateway comprises a Packet Data Network (PDN) Gateway (PGW). 25.A source gateway in a source Radio Access Network (RAN) to deliver datapackets for a session to a mobile station during inter-RAT (Radio AccessTechnology handover, the source gateway comprising: a receiverconfigured to receive one or more data packets for a bearer stream ofthe session from a network gateway; a control unit comprising a packetrouter configured to generate an empty GRE (Generic RoutingEncapsulation) packet responsive to an end-marker packet, saidend-marker packet indicating the end of the data packets for the bearerstream of the session; and a transmitter configured to: forward the datapackets to a target gateway in the target RAN; and send the empty GREpacket to the target gateway after the receiver receives the end-markerpacket.
 26. The source gateway of claim 25 wherein the receiver receivesthe end-marker packet for each of a plurality of bearer streams of thesession, and wherein the transmitter sends the empty GRE packet afterthe receiver receives the end-marker packet for each bearer stream. 27.The source gateway of claim 25 wherein the receiver receives theend-marker packet by receiving the end-marker packet sent to a sourceaccess node in the source RAN and returned to the source gateway by thesource access node, and wherein the packet router generates the emptyGRE packet responsive to the end-marker packet returned from the sourceaccess node.
 28. The source gateway of claim 25 wherein the sourcegateway comprises one of a 3^(rd) Generation Partnership Project (3GPP)serving gateway and a High Rate Packet Data (HRPD) serving gateway, thetarget gateway comprises the other of the 3GPP serving gateway and theHRPD serving gateway, and the network gateway comprises a Packet DataNetwork (PDN) Gateway (PGW).